Most speech conversations occur in the presence of competing sounds such as other voices or environmental noise. Hearing-impaired people often complain of difficulties understanding speech in noise backgrounds even if they do well in quiet. While physiological mechanisms underlying the direct masking of tones in the peripheral auditory system are well understood, this is not the case for more complex forms of masking involving spatial, temporal and cross-frequency interactions that are likely to be of central origin, and may degrade speech reception in the presence of competing sounds. We propose to investigate peripheral and central neural mechanisms underlying the masking of speech and other acoustic stimuli by measuring masked thresholds of single units in the auditory-nerve (AN) and inferior colliculus (IC) using the same stimuli and detection measures as in psychophysical experiments. Specific Aim 1 is to test the physiological validity of the notched-noise method used by psychophysicists to estimate auditory filter shapes from masking data in humans. Aim 2 is to investigate the neural basis for forward masking, which is minimal in the AN but, in the IC may be comparable to psychophysical forward masking. Aim 3 is to find neural correlates of the release from masking which occurs when additional sounds that are amplitude-modulated at the same frequency as a masker are introduced. Such "comodulation masking release" is thought to aid speech reception in the presence of interfering sounds that share a common pattern of modulation over broad frequency ranges. Aim 4 is to determine whether measures of neural masking for simple stimuli can be used to predict the masking of specific phonetic contrasts in speech. This research addresses fundamental issues in auditory theory such as neural mechanisms of auditory grouping, the roles of peripheral and central factors in masking, and the neural codes used for signal detection. It may lead to a better understanding of why hearing-impaired and elderly listeners have greater difficulties understanding speech among competing sounds than do normal listeners, and may help develop new kinds of hearing aids and auditory (cochlear and brainstem) implants that perform better in noisy environments.